Pros and Cons of Two Central Air-Conditioning Units for Two-Story Houses

Pros

• Having two systems allows you to strike a balance with the basic law of physics that "heat rises." The upper and lower units will have separate systems with separate thermostats so you control the temperatures for each level independently. Running two systems puts less strain and dependence on one unit to provide "total home" cooling. This can pay off in reduced wear and tear, potentially fewer break downs and an extended lifespan for each unit. However, in the unfortunate event that one system breaks down, you can stay cool on the level of the house where the functioning system provides cool air.

Cons

• Do not assume that having two systems will become "the great equalizer," with each system contributing 50 percent of your cooling needs. In most cases, the downstairs system will still contribute about 60 percent or more of the cooling, based again on the law of physics that heat rises. The other big "con" about having two systems is the initial installation cost. In addition to two outdoor condenser and compressor units, you will also have to install separate ductwork systems so that each system delivers cool air to its corresponding level of the home.

Two-System Equipment Considerations

• If you invest in two systems, one way to maximize the effectiveness is to go with two-stage air-conditioning units as opposed to single-stage air-conditioning systems. Single-stage systems are constantly on or off, which is why you feel temperature peaks and valleys. The exact is true of two-stage systems. Instead of being off or on, they use "intelligent technology" to provide even cooling. The motor starts in stage one, which is about at 60 percent of its capacity. The system only kicks into high gear, at 100-percent capacity, when the room requires stronger airflow for cooling. As a result, you use energy more efficiently and notice a more even level of cooling upstairs and downstairs.

Alternatives

• Zoning is one alternative to installing two separate systems. This involves identifying and setting up zones where cooling is needed most, such as in the upstairs bedrooms at night and in common areas such as the living room and dining room during the day. The dampers in the ductwork are adjusted so they can be manually opened and closed for summer cooling and winter heating. During the summer, the dampers are opened wider to force a greater amount of cool air through the ductwork to the various zones. An inexpensive alternative is to install two separate remote-control-operated thermostat systems -- one for upstairs and one for downstairs. At night, you can lower the temperature on the downstairs thermostat by remote control, and during the day, you can lower the temperature for the upstairs areas.